The sensor electrode comprises a closed sensor electrode housing with an electrolyte solution, for example, a pH buffer solution, a measuring electrode arranged in the electrolyte solution, and a gas bubble enclosed in the sensor electrode housing. The lower distal end of the sensor electrode housing is closed with an ion-selective sensor electrode diaphragm. The gas bubble is necessary to compensate for the temperature dependent volume change of the buffer solution.
Such sensor electrodes are described, for example, in DE 10 2008 055 084 A1 where the sensor electrode has an inner diameter smaller than 15.0 mm. When a sensor electrode of such a narrow diameter is turned upside down during transport, for instance, the gas bubble enclosed in the sensor electrode housing moves against the effect of gravity into the region between the sensor electrode diaphragm and the measuring electrode where it settles. When the sensor electrode is turned by 180° for assembly, the gas bubble remains in this region due to the phenomenon of surface tension. This trapping of the gas bubble has the effect that the electric connection between the sensor electrode diaphragm and the measuring electrode is interrupted. The problem of the gas bubble being trapped is conventionally overcome by carefully striking against the sensor electrode until the gas bubble is released and rises upward against the effect of gravity. No optical control is generally possible since the interior of the housing is not visible.
A sensor electrode is described in U.S. Pat. No. 4,838,999 wherein, in an attempt at reducing Ohmic resistance between two liquid volumes separated by a gas bubble, it is proposed providing a thread that soaks up electrolyte solution so that the electric resistance between the two liquid volumes is thereby drastically reduced. As an alternative, it is proposed providing a capillary channel that is integrated in the housing wall in the longitudinal direction of the housing and extending in the longitudinal direction. The manufacture of such a sensor electrode housing form is complex. The thread soaked with electrolyte solution may bridge the gas bubble, but cannot cause a flow of liquid under the gas bubble.